Chemistry problems.. dealing somewhat w/ conversions

These questions are from my AP chem assignment for this summer. Any help would be much appreciated! Thank you in advance!

first one..
"Mercury poisoning is a debilitating diesase that is often fatal. In the human body, mercury reacts with essential enzymes leading to irreversible inactivity of these enzymes. If the amount of mercury in a polluted lake is 0.4 micrograms Hg/mL, what is the total mass in kilograms of mercury in the lake? (The lake has a surface area of 100 mi^2 and an average depth of 20 ft.)"

second one..
"The contents of one 20. lb bag of topsoil will cover 10. square feet of ground to a depth of 1.0 inch. How many bags are needed to cover a plot which measures 200. by 300. m to a depth of 2.0 cm?"

I fortunately have the answers to the problems! (They were odd numbered problems, meaning they were in the back of the book, haha.)
Answer to #1 is 7 x 10^5 kg
Answer to #2 is 1.0 x 10^5 bags
Unfortunately, I'm not getting these results when I try the problems. Lol.
If you could show me the calculations, that'd be great
Thanks againnnnn!

These questions are from my AP chem assignment for this summer. Any help would be much appreciated! Thank you in advance!

first one..
"Mercury poisoning is a debilitating diesase that is often fatal. In the human body, mercury reacts with essential enzymes leading to irreversible inactivity of these enzymes. If the amount of mercury in a polluted lake is 0.4 micrograms Hg/mL, what is the total mass in kilograms of mercury in the lake? (The lake has a surface area of 100 mi^2 and an average depth of 20 ft.)"

second one..
"The contents of one 20. lb bag of topsoil will cover 10. square feet of ground to a depth of 1.0 inch. How many bags are needed to cover a plot which measures 200. by 300. m to a depth of 2.0 cm?"

I fortunately have the answers to the problems! (They were odd numbered problems, meaning they were in the back of the book, haha.)
Answer to #1 is 7 x 10^5 kg
Answer to #2 is 1.0 x 10^5 bags
Unfortunately, I'm not getting these results when I try the problems. Lol.
If you could show me the calculations, that'd be great
Thanks againnnnn!

Hmmm for problem #1 I get 6 x 10^5 kg, here are my steps (I only round at the very end)

1. The first one is simple. Find the volume of the lake. Within 1 mL there are 0.4 micrograms, therefore, within the total volume that you convert into ml, find the mass of mercury present. Just be careful with the units.